Mobile clients are often non-addressable or even unreachable from the public internet due to the Network Address Translation, firewall filtering and other network-related reasons. In such situations, the mobile client connectivity is asymmetric, since it is virtually one way: the client can initiate a connection to a server with a public internet address, but the server cannot initiate a connection to the client.
When it comes to delivering messages “instantly,” also known as “push” delivery, the client can “push” a message instantly to the server, but the server typically cannot do the same. If the server has a message to be sent to the mobile client, the server cannot send it immediately, but instead has to wait until the client first initiates a connection.